whit3rd wrote:> On Thursday, April 14, 2022 at 5:44:43 AM UTC-7, neo5...@gmail.com wrote: >> So which bjt's have such low leakage? >> Since Im thinking of it how much current can the collector base junction take without failing? > > It's not an easy question, low-leakage isn't a high test priority for BJTs. > At a guess, you'd want low-gain audio (low-frequency) parts, which have relatively > high base doping. > > How about a zener diode? Should be good for high currents, certainly specified for 'em. >The best ones I know about are BFT25As (RIP, sniff sniff), which are small-geometry 5-GHz NPNs. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
Max current for a JFET connected as diode
Started by ●April 13, 2022
Reply by ●April 14, 20222022-04-14
Reply by ●April 14, 20222022-04-14
Phil Hobbs wrote:> whit3rd wrote: >> On Thursday, April 14, 2022 at 5:44:43 AM UTC-7, neo5...@gmail.com wrote: >>> So which bjt's have such low leakage? >>> Since Im thinking of it how much current can the collector base >>> junction take without failing? >> >> It's not an easy question, low-leakage isn't a high test priority for >> BJTs. >> At a guess, you'd want low-gain audio (low-frequency) parts, which >> have relatively >> high base doping. >> >> How about a zener diode? Should be good for high currents, certainly >> specified for 'em. >> > > The best ones I know about are BFT25As (RIP, sniff sniff), which are > small-geometry 5-GHz NPNs.See, e.g. this thread from a dozen years ago: "Types of diode-connected BJTs" <https://groups.google.com/g/sci.electronics.design/c/wonzon1fZGI/m/f31wyYfP4aUJ> Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
Reply by ●April 14, 20222022-04-14
On Thursday, April 14, 2022 at 1:20:21 PM UTC-7, John Larkin wrote:> On Thu, 14 Apr 2022 12:20:31 -0700 (PDT), whit3rd <whi...@gmail.com> > wrote: > >On Thursday, April 14, 2022 at 5:44:43 AM UTC-7, neo5...@gmail.com wrote: > >> So which bjt's have such low leakage? > >> Since Im thinking of it how much current can the collector base junction take without failing? > > > >It's not an easy question, low-leakage isn't a high test priority for BJTs. > >At a guess, you'd want low-gain audio (low-frequency) parts, which have relatively > >high base doping.> Small geometry RF transistors seem to have fA reverse currents when > used as diodes. I'd expect audio parts to be much bigger.True, but there's the 'how much current' question to consider; small geometry isn't always a good tradeoff, if it's a protection diode application.> Note that my diode connection exploits reverse beta to keep the base > current down, so it should be good for rated max Ic. That will limit > max reverse voltage.The tradeoff with RF transistors includes low Rbb, so keeping base current down might not be an issue. Low-noise audio requires high current gain forward, which implies low base doping and high Rbb is acceptable. Those wouldn't be a great choice for protection.
Reply by ●April 14, 20222022-04-14
On Thu, 14 Apr 2022 16:25:02 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>whit3rd wrote: >> On Thursday, April 14, 2022 at 5:44:43 AM UTC-7, neo5...@gmail.com wrote: >>> So which bjt's have such low leakage? >>> Since Im thinking of it how much current can the collector base junction take without failing? >> >> It's not an easy question, low-leakage isn't a high test priority for BJTs. >> At a guess, you'd want low-gain audio (low-frequency) parts, which have relatively >> high base doping. >> >> How about a zener diode? Should be good for high currents, certainly specified for 'em. >> > >The best ones I know about are BFT25As (RIP, sniff sniff), which are >small-geometry 5-GHz NPNs. > >Cheers > >Phil HobbsAny guesses about SiGe transistors as diodes? I should get some and try them. One figure of merit is capacitance * reverse current. -- I yam what I yam - Popeye
Reply by ●April 14, 20222022-04-14
On Thu, 14 Apr 2022 16:32:50 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>Phil Hobbs wrote: >> whit3rd wrote: >>> On Thursday, April 14, 2022 at 5:44:43 AM UTC-7, neo5...@gmail.com wrote: >>>> So which bjt's have such low leakage? >>>> Since Im thinking of it how much current can the collector base >>>> junction take without failing? >>> >>> It's not an easy question, low-leakage isn't a high test priority for >>> BJTs. >>> At a guess, you'd want low-gain audio (low-frequency) parts, which >>> have relatively >>> high base doping. >>> >>> How about a zener diode?�� Should be good for high currents, certainly >>> specified for 'em. >>> >> >> The best ones I know about are BFT25As (RIP, sniff sniff), which are >> small-geometry 5-GHz NPNs. > >See, e.g. this thread from a dozen years ago: > >"Types of diode-connected BJTs" > ><https://groups.google.com/g/sci.electronics.design/c/wonzon1fZGI/m/f31wyYfP4aUJ> > >Cheers > >Phil HobbsI miss JT's harassment. He was good at it. -- I yam what I yam - Popeye
Reply by ●April 14, 20222022-04-14
> >> The best ones I know about are BFT25As (RIP, sniff sniff), which are > >> small-geometry 5-GHz NPNs.Re BFT25 (without the A, has higher Noise Figure, lower f_T) about 1700, left in stock at DigiKey https://www.digikey.com/en/products/base-product/nxp-usa-inc/568/BFT25/35932 Though I assume if one chose another of similar construction & specs, they should see similar results (?) if not in the ballpark... = RS
Reply by ●April 15, 20222022-04-15
jlarkin@highlandsniptechnology.com wrote:> On Thu, 14 Apr 2022 16:32:50 -0400, Phil Hobbs > <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> Phil Hobbs wrote: >>> whit3rd wrote: >>>> On Thursday, April 14, 2022 at 5:44:43 AM UTC-7, neo5...@gmail.com wrote: >>>>> So which bjt's have such low leakage? >>>>> Since Im thinking of it how much current can the collector base >>>>> junction take without failing? >>>> >>>> It's not an easy question, low-leakage isn't a high test priority for >>>> BJTs. >>>> At a guess, you'd want low-gain audio (low-frequency) parts, which >>>> have relatively >>>> high base doping. >>>> >>>> How about a zener diode? Should be good for high currents, certainly >>>> specified for 'em. >>>> >>> >>> The best ones I know about are BFT25As (RIP, sniff sniff), which are >>> small-geometry 5-GHz NPNs. >> >> See, e.g. this thread from a dozen years ago: >> >> "Types of diode-connected BJTs" >> >> <https://groups.google.com/g/sci.electronics.design/c/wonzon1fZGI/m/f31wyYfP4aUJ> >> >> Cheers >> >> Phil Hobbs > > I miss JT's harassment. He was good at it.But when Fields got involved, it went to a whole new level. ;) Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
Reply by ●April 15, 20222022-04-15
Rich S wrote:>>>> The best ones I know about are BFT25As (RIP, sniff sniff), which are >>>> small-geometry 5-GHz NPNs. > > > Re BFT25 (without the A, has higher Noise Figure, lower f_T) > about 1700, left in stock at DigiKey > https://www.digikey.com/en/products/base-product/nxp-usa-inc/568/BFT25/35932 > > Though I assume if one chose another of similar > construction & specs, they should see similar results (?) > if not in the ballpark... > = RSOh, I have three reels of BFT25As in stock. Doesn't help much with customer designs. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
Reply by ●April 15, 20222022-04-15
jlarkin@highlandsniptechnology.com wrote:> On Thu, 14 Apr 2022 16:25:02 -0400, Phil Hobbs > <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> whit3rd wrote: >>> On Thursday, April 14, 2022 at 5:44:43 AM UTC-7, neo5...@gmail.com wrote: >>>> So which bjt's have such low leakage? >>>> Since Im thinking of it how much current can the collector base junction take without failing? >>> >>> It's not an easy question, low-leakage isn't a high test priority for BJTs. >>> At a guess, you'd want low-gain audio (low-frequency) parts, which have relatively >>> high base doping. >>> >>> How about a zener diode? Should be good for high currents, certainly specified for 'em. >>> >> >> The best ones I know about are BFT25As (RIP, sniff sniff), which are >> small-geometry 5-GHz NPNs. >> >> Cheers >> >> Phil Hobbs > > Any guesses about SiGe transistors as diodes? I should get some and > try them. One figure of merit is capacitance * reverse current. > > >Haven't tried it. I should hang some on my nice shiny HP 4145B and find out. I use BFU520As to replace BFT25As for actual three-terminal jobs. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
Reply by ●April 15, 20222022-04-15
On Thursday, April 14, 2022 at 3:20:35 PM UTC-4, whit3rd wrote:> On Thursday, April 14, 2022 at 5:44:43 AM UTC-7, neo5...@gmail.com wrote: > > So which bjt's have such low leakage? > > Since Im thinking of it how much current can the collector base junction take without failing? > It's not an easy question, low-leakage isn't a high test priority for BJTs. > At a guess, you'd want low-gain audio (low-frequency) parts, which have relatively > high base doping. > > How about a zener diode? Should be good for high currents, certainly specified for 'em.A zener is too leaky for this application alone. It is a Zener circuit but where where I'm adding the extra forward voltage drop of a low leakage diode in series so the zeners leakage current isnt causing error on the signal line below the zener clamp voltage. Jfet clamps are very good at staying out of the way in terms of their leakage. I've used them for many microphone or other audio signal line protection but never had to pay much attention to the current through them when conducting. - Just that it does when called for. This application is a little different. This is to protect a "universal" voltage input where the range of signal can be 0 to 10 mV or a large as 0 to 10V. But also prevent damage when some dope connects the +24V power supply rail to it. AofE does have the closest thing I've read that discusses some of the question but doesn't really speak to the question of the limit of current through the clamp. Logically is would be below the max rating for the device but how far below?
